1. Field of the Invention
This invention relates to relays for protecting an electrical power transmission system comprising a two or more ended electrical feeder. In particular the invention relates to differential relays which operate to protect the system when the difference between the value of an electrical quantity, usually current, monitored at two or more different points on the feeder exceeds a predetermined amount.
2. Description of Related Art
In recent years differential relays have been developed which use digital data transmission to transmit the necessary data regarding the monitored electrical quantities between the monitoring points on the feeder. Such differential relays are generally more reliable and faster than relays using analog tansmission. Differential relays using digital data transmission do, however, suffer the disadvantage that as digital data, unlike continuous analog signals, represent the value of the monitored electrical quantity at discrete time intervals, it is necessary that some form of synchronisation be provided so that digital data collected at different points on the feeder can be aligned to the same time instant. Known differential relays using digital data transmission solve this synchronisation problem by providing an external radio clock, but this leads to additional cost and complexity, and furthermore a suitable radio clock may not be available to all users of the power transmission system. Other known differential relays using digital data transmission solve the synchronisation problem by using a clock signal derived from the communication multiplexing equipment, but this approach suffers the problem that the derivation of the clock signal is very dependent on the actual design of the communication multiplexing equipment, the network configuration and the control hierarchy adapted: as the relay would have to work with a variety of communication equipment, the cost for developing different clock interfaces could be prohibitive. Furthermore, a clock source may not be available if the relay is to be used over a direct non-multiplexing link.
In U.K. Patent Application No. GB 2072974A there is described a differential relay using digital data transmission in which independent clocks are provided at the monitoring points along the line to be protected, the clocks defining the time intervals at which electrical data is measured at the monitoring points. Digital data relating to the current measured at a local monitoring point is transmitted to a remote monitoring point and stored in a first memory, while digital data relating to the current measured at the remote monitoring point is stored in a second memory. A shift register is arranged to shift through the data in the first memory to correct the data therein by a factor corresponding to the transmission time of the data collected at the local monitoring point between the local and remote monitoring points. The data collected at sampling times t.sub.1 +.alpha., t.sub.2 +.alpha., t.sub.3 +.alpha.--at the remote monitoring point is then compared with the correct data collected at sampling times t.sub.1, t.sub.2, t.sub.3 --at the local monitoring point, where .alpha. is the time interval between the sampling times at the two points, to determine whether the relay is to operate.
Such a relay suffers the disadvantages however that in order to achieve a reasonable amount of accuracy, a very high sampling rate must be used for the derivation of data at the local monitoring point.